Measured loads in centrifuge tests on piled embankments were compared with values calculated by three analytical arching models. The Zaeske and Van Eekelen models use sets of arches that give the direction of the load transfer, and result in an increasing load on the soft soil with increasing embankment thickness or surcharge load, which matches the measurements. The Svanø model, however, assumes that the load on the soft soil remains constant with increasing embankment thickness or surcharge load, thereby being transferred directly to the piles. Therefore, the Zaeske and Van Eekelen models match the measurements better than the Svanø model. The Van Eekelen model slightly overestimates the pile efficiency for relatively thin embankments, but matches the measurements better than the Zaeske model for increasing embankment thickness or surcharge load.

This paper presents numerical and analytical models to complement experimental data obtained from 40 centrifuge tests of geosynthetic-reinforced piled embankments, 12 of which had a surcharge applied at the embankment surface. The main parameters of study were pile diameter, pile spacing, embankment height and geosynthetic stiffness. The tests measured the force transferred to the piles, the embankment surface settlements and the maximum geosynthetic reinforcement deflections below the embankment under conditions without any support from the underlying soil. Maximum geosynthetic deflections measured longitudinally between piles were then compared with values predicted by European design guidelines. A 3D numerical model, initially validated by centrifuge tests, was then used to compute those geosynthetic tensile forces not measured in the reduced-scale physical models. Numerical values of geosynthetic tensile forces were also compared with corresponding values predicted by European guidelines.

A programme of 20 centrifuge tests was carried out on piled embankments without reinforcement to assess the influence of geometric parameters on surface settlement and on the load transfer mechanism. The three main geometric parameters studied were pile spacing, embankment height and area ratio, which were varied in ranges commonly adopted in practice. The measurements of the forces on the piles allowed assessment of the load transfer mechanisms, and thus the efficiency, which were shown to increase with embankment height and area ratio. Subsoil stress between piles was also assessed and compared with literature data. The centrifuge model test results for stress on the subsoil at the point of ‘maximum arching’ presented good agreement with the method proposed by Hewlett and Randolph in 1988. Measurements of the surface settlements allowed evaluation of the critical height at which surface differential settlements did not occur. The values of critical height obtained herein are in good agreement with recent literature recommendations.

Soft ground improvement techniques have evolved substantially in Brazil in recent years. However, their application in soft and very soft clays requires a good understanding of the fundamentals of ground improvement techniques suited to the problem as well as the actual field behavior when implemented on a real scale. This paper describes some of the most widely used ground improvement techniques in the context of very soft clays in Brazil. The techniques described in the paper use prefabricated vertical drains (PVD) such as vacuum preloading; or combine PVD and rigid inclusion, such as CPR grouting; or are purely column-like elements such as piled embankments (including those executed with the deep mixing technique, DSM); or combine column-like elements with the drainage function, such as stone columns and geosynthetic encased columns; or use cementitious binders such as shallow soil mixing. The paper reference condition is a soft clay foundation in which no strengthening is implemented, such as, an embankment with basal reinforcement or soft clay with vertical prefabricated drains, or the use of vacuum preloading to speed up the consolidation rate. The applications of the ground improvement techniques are illustrated by case histories, numerical analyses, or physical models. Different types of measurements are used to evaluate the performance of each technique, including settlements, horizontal displacements, excess pore pressures, embankment applied stresses, stress concentration factors, and clay strength following the ground treatment. The settlement improvement factor β, the ratio between the settlements for untreated and treated conditions, is shown to be a suitable parameter to assess the degree of improvement imposed in the soft foundation by ranking the various methods in increased order of strengthening effect.